Internal combustion (IC) engines generate torque by combusting a fuel and air mixture. The torque not only provides propulsion force to the wheels but also drives auxiliary engine loads. For example, the torque is used to drive loads including, but not limited to, an A/C compressor, a generator or alternator, a coolant pump, an oil pump and the like. In the event of an unanticipated load increase, the engine speed decreases and the engine may stall.
To prevent engine stall, engine control systems of spark ignited IC engines maintain a torque reserve by regulating spark timing to a less than optimal amount. Because this spark advance is sub-optimal for a given fuel/air rate, the engine produces less torque than at the optimal timing (i.e. minimum spark for best torque (MBT)). If additional torque is quickly needed to drive an increased load (i.e., faster than the A/F ratio can be changed), the spark timing is advanced closer to optimal to produce additional torque for the same air/fuel rate. Likewise, if less torque is needed, spark timing is retarded further from MBT. This method, however, results in the engine generally running at sub-optimal spark timing to maintain the desired torque reserve,
Another method of preventing engine stall is to regulate the engine load. For example, the load (i.e., torque) of the generator can be regulated for idle speed control. In this manner, the engine control system can maintain the spark timing closer to MBT to reduce fuel consumption at idle. Reducing the generator load achieves the same result as increasing engine torque by advancing spark. Tight control the amount of torque load added or subtracted by the generator is required in order to achieve a combination of smooth (i.e. driver transparent) control and improved fuel consumption. However, traditional methods of estimating generator torque are not sufficiently accurate to provide the tight control required. For example, traditional methods of estimating generator torque, when reflected to the engine, can have an error of approximately +/−20 Nm.